Those skilled in the hydrocarbon recovery industry understand that pumps at the lower end of wells are conventionally used to pump oil to the surface via production tubing positioned within a well casing. The pump is typically powered at the surface, with the power being transmitted through a rod string positioned within the production tubing. A rod string conventionally has been reciprocated to drive the downhole pump, although a progressing cavity pump driven by a rotating rod is being increasingly used, particularly in wells producing heavy, sand-laden oil or producing fluids with high water oil ratios.
Whether the rod which drives the pump (the sucker rod) reciprocates or rotates, the rod generally is guided so that it does not rub against the interior walls of the production tubing, and thus cause excessively wear on either the sucker rod, the sucker rod couplings, or the production tubing. In practice, sucker rods and production tubing almost never hang perfectly concentric within a well. Moreover, few if any wells produce crude oil free of abrasives and water, and those contaminants increase wear if the sucker rod string contacts the inside of the production tubing. Whether the pump driving system utilizes a reciprocating or a rotating rod, tubing wear and rod wear accelerate as production rates, hole deviations, water/oil ratios, and sand concentrations increase. While rod guides traditionally have thus long been used to generally center the rod within the production tubing, the need for improved rod guides increases with the changing variables discussed above.
Many rod guides have a plurality of radially outward projecting fins, ribs, or vanes with a fin outer diameter (O.D.) close to the internal diameter (I.D.) of the production tubing, so that the fins achieve a maximum standoff between the rod coupling and the tubing. The cross-sectional area or annular spacing between the rod guide and tubing, coupled with the length to diameter (L/D) ratio of the rod guide, and the shape and smoothness of the rod guide, determines the undesirable pressure drop across this type of guide, which must be overcome by the downhole pump. Other rod guides are unfinned and have a generally cylindrical outer body, with the body having an O.D. smaller than the I.D. of the tubing. The difference or annular space between the maximum O.D. of the guide body and tubing I.D., coupled with the L/D ratio and the shape and smoothness of the rod guide, determines the pressure drop across this type of rod guide. Since the standoff between the rod couplings and the tubing is also less for this latter type of rod guide, unfinned rod guides have a disadvantage of less erodible wear volume (EWV) to prevent metal-to-metal contact between the sucker rod or rod couplings and the production tubing, and thus tinned rod guides are often favored by oil recovery operators.
Rod guides are traditionally spaced along the length of a rod string to prevent the rod string from engaging the tubing string. To maintain the rod guides at their desired spacing along the sucker rod, rod guides installed in the field are manufactured using plastic, rubber, and metal. Various designs are used to create a frictional grip on the rod in order to secure the rod guide in position. Field installed rod guides (FIGS) traditionally do not maintain their desired gripping engagement with the rod over a long period of time, particularly when high axial forces are encountered by the rod guide and when increasingly more power is transmitted from the surface to the downhole pump through the rod. While it is thus desirable that a rod guide be installed at the well site or at a location convenient to the well operator, FIGS traditionally are not able to achieve reliable engagement with the rod. Other versions of FIGS utilize a rubber guide body with a metal C-spring molded within the rubber body to supply a supplemental force which increases the frictional grip of the guide to the rod, as disclosed in U.S. Pat. No. 4,928,472. This latter type of rod guide is typically unfinned and has a high pressure drop, and generally is also poor at reliably securing the guide to the rod.
Rod guides manufactured from plastic have been molded directly onto the rod. These molded-on rod guides, as disclosed in U.S. Pat. No. 4,088,185, thus have the advantage of more reliably engaging the rod to maintain the rod generally concentric within the tubing string. Molded-on rod guides are also relatively inexpensive to manufacture, although these prior art rod guides have the disadvantage of practically requiring that the entire rod be sent from the field to a molding facility to remove a worn-out guide and mold on a new guide, after which the rod with new guides may then be returned to the field. A rod guide with a diagonal slot designed for maintaining a guide on a rod is disclosed in U.S. Pat. No. 3,442,558, while a similar snap-on guide and scraper is disclosed in U.S. Pat. No. 3,282,344. A field installable rod guide is disclosed in U.S. Pat. No. 4,858,688.
A significant problem with rod guides concerns balancing the opposing desires of maximizing the life of the rod guide (which is related to the erodible wear volume), while also minimizing the pressure drop across the rod guide which the pump must overcome to transmit the fluids to the surface. For a tinned or ribbed rod guide, the life of the rod guide is enhanced by providing thick ribs which produce a substantial wear area for the guide to contact the tubing string. The more erodible wear volume (EWV) for a rod guide, the longer the rod guide is likely to last in the field, although such increased erodible wear volume also undesirably increases the pressure drop across the rod guide. A significant advantage is achieved by maximizing these factors in the manner described in U.S. Pat. No. No. 5,115, 863.
U.S. Pat. No. 5,119,876 discloses one version of a rod guide including a cylindrical centralizer body which hinges open during a spreading operation for insertion on a mount provided on the rod guide shank. The centralizer body is returned to its cylindrical shape after installation, and the centralizer body is welded to maintain its desired cylindrical form while on the rod. This type of rod guide has not proven to result in long life, and the operation of bonding the split body to its desired cylindrical form after installation is a drawback to easy field serviceability.
Improved rod guides and methods for installing such rod guides are thus desired by oil recovery operators to meet the demands of operators for fluid recovery systems which can operate at high production rates, which can operate in substantially deviated holes, which can reliably recover hydrocarbons with high water/oil ratios, and/or which can recover fluids contaminated with sand or other abrasives. The disadvantages of the prior art are overcome by the present invention, and an improved a rod guide, a method of installing a rod guide, and a test unit for testing a rod guide are hereinafter disclosed.